1. Field of the Invention
The present invention concerns an x-ray computed tomography apparatus of the type having a stationary x-ray detector that at least partially encloses an examination volume in one plane; and a stationary device for generation of x-ray radiation, the stationary device including an x-ray source that extends annularly over an angle of at least 180° around the examination volume; and including one or more light scanning units with which an x-ray focus moving along the x-ray source can be generated by scanning of the x-ray source; from which x-ray focus an x-ray beam is directed through the examination volume onto respective, momentarily opposite detector elements of the stationary x-ray detector.
2. Description of the Prior Art
Computed tomography apparatuses are used, for example, in medical imaging in order to acquire images of the inside of the body of a patient. A computed tomography apparatus includes, among other things, a device for generation of x-ray radiation, an x-ray detector and a patient positioning table with which the examination subject can be moved through the examination volume along a system axis (known as the Z-axis) during the examination. The device for generation of x-ray radiation generates an x-ray beam that emanates from an x-ray focus that rotates around the examination volume. In examinations the x-ray beam, expanded in a fan shape in a slice plane of the examination volume (X-Y plane) perpendicular to the system axis, penetrates a slice of the examination subject, for example a body slice of a patient, and strikes the detector elements of the x-ray detector situated opposite the x-ray focus. The angle at which the x-ray beam penetrates the body slice of the subject and, if applicable, the position of the patient positioning table normally varies continuously during the image acquisition with the computed tomography apparatus.
In computed tomography apparatuses of the third generation the rotating x-ray focus is generated by an x-ray tube that, like the x-ray detector, is fastened on a rotary frame (gantry) that is rotatable around the examination volume. The rotation speed of the rotary frame in recent years has been steadily increased in order to achieve faster scan speeds in the image acquisition. For reasons of mechanical stability and safety, however, in computed tomography apparatuses of the third generation a limit has been reached that, due to the masses to be moved and the high acceleration forces resulting therefrom, no longer allows a significant increase of the rotation speed of the rotary frame.
In computed tomography apparatuses of the fourth generation the x-ray detector is arranged as a stationary ring around the examination volume so that only the x-ray tube must still be moved with the rotary frame. Here as well, however, significant forces that limit the rotation speed act on the x-ray tube so a further increase of the rotation speed of the rotary frame is not likely.
To avoid this problem, computed tomography apparatuses of the fifth generation have become known in which both the device for generation of x-ray radiation and the x-ray detector are stationary. In these computed tomography apparatuses an x-ray target is used that at least partially encloses the examination volume of the computed tomography apparatus in one plane. An x-ray focus from which the x-ray radiation emanates is generated on this target, the x-ray focus moving around the examination volume. These computed tomography apparatuses thus operate entirely without a mechanically moving x-ray tube. The x-ray target extends either completely around the examination volume or at least over an angle of more than 180°. In the same way the x-ray detector encloses the examination volume either completely or over an angle of at least 180° and is arranged such that the x-ray beam emanating from a moving x-ray focus passes through the examination volume onto respective, momentarily opposite detector elements of the stationary x-ray detector.
For example, U.S. Pat. No. 4,352,021 discloses a computed tomography apparatus of the fifth generation in which the x-ray target and the x-ray detector respectively surround the examination volume over an angle of approximately 210° For generation of the x-ray focus, an electron beam is generated with an electron gun and is directed over the x-ray target by suitable deflection. Due to numerous disadvantages, however, this technique (also known under the abbreviation EBCT (Electron Beam Computed Tomography)) has previously not found significant use in clinical application. A tilting of the examination plane in such systems is not possible. Like the accessibility of the patient for the operator, the available region for the horizontal positioning of the patient is severely limited. Such an x-ray computed tomography apparatus requires a very large examination room. Due to the long path of the electron beam, instabilities of the focus as well as a larger focus diameter occur, causing the spatial resolution to be impaired. The complex electron beam optics and long setup times lead to a lower reliability and lower patient throughput.
To avoid the problem associated with the long electron beam, U.S. Pat. Nos. 6,181,765 and 6,731,716 disclose x-ray computed tomography apparatuses of the fifth generation wherein an annular x-ray tube is used in which a number of thermal electron emitters are arranged distributed around the ring. Such thermal emitters, however, require a high electrical power in order to keep them at the required temperature during a scan. A comparable arrangement in which a field emission source is used as an electron source is known from United States Patent Application Publication No. 2002/0094064 A1. The individual regions of this cold electron source can be selectively addressed by an attached electrode structure in order to be able to emit electrons locally by means of the local electrical field. In these x-ray tubes the field emission current is controlled by the voltage applied at the electron source and not by the temperature, as in the thermal electron emitters. Cold electron sources are not yet able to generate the power/current densities required for many computed tomography apparatus applications with an acceptable lifespan. The electrical control of the many thousands of emitters arranged in an x-ray tube is additionally very costly.
U.S. Pat. No. 4,606,061 discloses a further embodiment of a computed tomography apparatus of the fifth generation in which an x-ray focus moving around the examination volume is generated on an x-ray target completely annularly surrounding the examination volume. An electron source ring that is coaxial with the x-ray target is likewise provided that is controlled by a laser beam directed with a laser scanning unit over its surface for electron emission. The laser scanning unit is arranged on the central ring axis of the electron source ring in order to be able to symmetrically scan this over an angle of 360°. Even this technique, however, does not avoid all problems associated with the EBCT technique. The scanning beam path must be shielded in such a system in order to avoid an unwanted interruption by the operator or other objects within the examination room. This also clearly limits the region available for a horizontal displacement of the patient as well as the accessibility of the patient for an operator.